Wow. I might get myself one of those for Bumpy's tank, her current filter has a strong current but weak intake. It's inconvenient, because I have to do water changes more often.

Oh, try bullfrog water. They eat a lot, and produce tons of bio-waste.

Some updated pictures of the tank could be handy.

My frogs tend to stay in one spot the whole day, but at night it's an entirely different story. I'm not sure about FBT's, I've heard they're a bit less skittish. Well, Bumpy is the skittish frog of the pair. Gnag just usually don't care if I'm there, she only cares about that juicy cricket she's eyeing on the lily pad

Well, my tactic for water changing is to get a bucket, place it beside or on top of the tank, reach in and scoop small portions of water out with a cup. Turn the filter off before-hand. Also, splashing water down a bit by emptying the regular portion during the water change, then pouring the fresh dechlorinated water from the jug/bucket/etc onto the gravel can help get gunk and other unwanted matter out from the gravel, so it is sucked up by the filter. I use this strategy a lot to help clean the smooth stones. Only when the frogs are hiding, though, I try not to disturb them when they are out and about.

I don't think the deep-down gravel is a concern. If your FBT's are anything of the swimmer that my frogs are, they'll be fine.

The moss is only a danger if they feed on it. If they ingest it, it can cause prolapse, a very scary thing I have yet to encounter. If you're placing it somewhere far away from the feeding area and not a place where crickets could get to and the frogs go to, you could add it as decoration. I do so for part of my tank, but I always steer crickets away from that zone during feeding.